The present invention relates to semiconductor laser devices and optical pickup devices, for example, to a semiconductor laser device and an optical pickup device which are suitable for use in an optical disk apparatus which optically records or reproduces information to or from information recording mediums such as optical disks.
With a view to smaller size, thinner thickness and higher reliability of optical pickup devices, there has been an optical pickup device equipped with a hologram element. FIG. 8 shows an example of a semiconductor laser device equipped with a hologram element to constitute an optical pickup device.
In this semiconductor laser device, a laser beam 107 emitted by a semiconductor laser chip 101 is first diffracted by a grating element 103. It is noted that the semiconductor laser chip 101 is placed on a stem 106, and the grating element 103 is placed on a hologram element/grating element positioning cap 105. Out of the diffracted light diffracted by this grating element 103, a 0th-order diffracted beam 108A is used for signal detection and ±1st-order diffracted beams 108B, 108C are used for tracking signal detection.
However, the 0th-order diffracted beam 108A and ±1st-order diffracted beams 108B, 108C diffracted by this grating element 103, during passage through a hologram element 104 on a forward way lead to a disk (not shown), which is an object of irradiation, are diffracted by the hologram element 104. Then, only a 0th-order diffracted beam 109 by this hologram element 104 can be used as a signal. In addition, ±1st-order diffracted beams 110A, 110B by the hologram element 104 result in loss components.
The 0th-order diffracted beam 109 by the hologram element 104 is diffracted again by the hologram element 104 on the backward way from the disk, becoming a 1st-order diffracted beam 111, which is converged to a photoreceptive portion of a photodetector 102.
It is well known that, on the backward way from the disk, the 0th-order diffracted beam that has passed through the hologram element 104 is not only useless for signal detection, but makes a cause of noise due to its return to the laser chip 101.
Also, in an optical pickup device equipped with the above-described semiconductor laser device, there would arise problems as follows.
On the forward way, none of the ±1st- and higher-order diffracted beams diffracted at the passage of the holographic pattern of the hologram element 104 lend themselves to signal detection. That is to say, they make a loss component. Therefore, it is necessary to increase the power of the laser beam emitted by the laser chip 101 to an extent corresponding to the loss component.
As a result, the ratio of the 0th-order diffracted beam in the hologram element 104 would necessarily be set to a high one, whereas, in this case, the diffraction efficiency in the hologram element 104 on the backward way would inevitably become lower, so that the signal intensity by the diffracted beam diffracted toward the photoreceptor 102 would inevitably become lower. Further, since the hologram element 104 has a high ratio of the 0th-order diffracted beam, the ratio of the backward beam from the disk to the laser chip 101 would also be high, making a cause of noise as described above, which leads to a difficulty in use.
As it stands, the write speed onto the disk is constrained primarily by laser power. Therefore, on the forward way, the write speed is limited by the loss of light diffracted by the holographic pattern of the hologram element 104, and a reduction in optical loss on the forward way is required to achieve higher-speed write operations.